For tetra-lateral position sensitive detectors (PSD), the calculated positions obtained with the conventional equations of position calculation do not correspond to the real spot positions., and have serious pincushion distortion. A modified equation is proposed in terms of the analytical solutions of Lucovsky equation. According to the results, the correction of the PSD can be easily achieved and the linearity can also be greatly improved in most of the central active area by the modified equation. In the 60% of the active area of 1×1 cm 2 PSD, the position root mean square (rms) error was less than 3 μm , the position non-linearity was less than 0.05%. This modified equation can be fit for all tetra-lateral PSDs.